Small microphones that can be manufactured with low cost are highly desirable components in many portable electronic products. In current design approaches, however, the small size of the microphone results in diminished sensitivity to sound, and in particular poor sensitivity to low frequencies. As a result, great care must be taken in the design to maximize sensitivity, which generally adds to the complexity and cost of the device.
The conventional approach to creating small microphones is to fabricate a thin, lightweight diaphragm that vibrates in response to minute sound pressures. The motion of the diaphragm is usually transduced into an electronic signal through capacitive sensing, where changes in capacitance are detected between the moving diaphragm and a fixed backplate electrode. As the size of the diaphragm is reduced, however, in an attempt to make a small, low-cost microphone, the stiffness of the diaphragm is generally increased. This increased stiffness causes a marked reduction in its ability to deflect in response to fluctuating sound pressures. This increased stiffness with decreasing size is a fundamental challenge in the design of small microphones. An additional challenge in the design of microphones comes from the use of a backplate electrode to achieve capacitive sensing. To obtain an electronic readout, it is necessary to apply a biasing electric voltage between the backplate and the diaphragm. This will result in a force that is proportional to the square of the voltage (and hence is independent of its polarity) that always acts to attract the flexible diaphragm toward the fixed backplate. Because the output of the electronic circuit will be proportional to the biasing voltage used, one is tempted to use as high a voltage as possible to increase sensitivity. However, great care must be taken to ensure that the resulting attractive force is not sufficient to collapse the diaphragm into the backplate. To avoid this potentially catastrophic situation, one may use a diaphragm that has a higher stiffness so it can resist the attractive force, but this also results in reduced acoustic sensitivity. Achieving a compromise between increased electronic sensitivity through the use of a high bias voltage and avoiding diaphragm collapse is one of the most challenging aspects of microphone design.
Because microphones are generally designed to respond to sound pressures using a pressure-sensitive diaphragm, it is important to ensure that the pressure due to sound acts on only one side, or face of the diaphragm otherwise the pressures acting on the two sides will cancel. (In some cases, this cancellation property is used to advantage, especially where the microphone can be designed such that undesired sounds are cancelled while desired sounds are not). In addition, because the diaphragm is also subjected to relatively large atmospheric pressure changes, it is important to incorporate a small vent to equalize static pressures on the two sides of the diaphragm. Depending on the size of the enclosure around the back-side of the diaphragm and the size of the pressure-equalizing vent, the low-frequency response of the diaphragm will also be reduced by the vent. In small microphones, the air volume behind the diaphragm is generally quite small and as a result, motion of the diaphragm can cause a significant change in the volume of the air. The air thus becomes compressed or expanded as the diaphragm moves, which results in a respective increase or decrease in its pressure. This pressure creates a restoring force on the diaphragm and could be viewed as an equivalent linear air spring having a stiffness that increases as the nominal volume of air is reduced. The combined effects of the diaphragm's mechanical stiffness, the pressure-equalizing vent, and the equivalent air spring of the back volume need to be considered very carefully in designing microphones that are small, have good sensitivity and respond at low audio frequencies
When a microphone is sensing small differences in the air pressure (i.e., sound waves), both large and small diaphragms will, in principle, be equally capable of picking up low frequencies. The lower limiting frequency (LLF) of a pressure microphone is typically controlled by a small pressure equalization vent that prevents the microphone diaphragm from responding to changes in the ambient barometric pressure. The vent typically acts as an acoustic low cut filter (i.e., a high-pass filter) whose cut-off frequency depends on the vent dimensions (e.g., diameter and length). As a sound pressure wave passes the microphone, longer wavelengths (lower frequencies) will tend to equalize pressure around the diaphragm and thus cancel their response.